1. Field of the Invention
The present invention relates to a hydraulic system for construction equipment that can perform ground leveling using an excavator.
More particularly, the present invention relates to a hydraulic system for construction equipment having a float function, which can perform ground leveling as making a boom descend due to its own weight without using hydraulic fluid that is discharged from a hydraulic pump.
2. Description of the Prior Art
Generally, in the case of performing the ground leveling work using an excavator, the primary purpose of a float valve is to return hydraulic fluid to a hydraulic tank by making flow paths of a large chamber side and a small chamber side of boom cylinders communicate with each other during a boom-down operation.
In this case, the boom descends due to its own weight, and is moved up and down depending upon the shape of the ground by the operation of an arm in a state where each hydraulic cylinder carries a low load to facilitate the ground leveling work. Also, the hydraulic fluid that is discharged from the hydraulic pump can be used for other working devices, and thus energy can be saved.
A hydraulic system having a float function is provided with an anti-drop valve connected to the boom cylinder to prevent the drop of the boom, a float valve, and a main valve. Due to this construction, it is difficult to match the three valves in the equipment, and hydraulic pipes for connecting the valves are increased to cause the increase of the manufacturing cost.
As shown in FIG. 1, a hydraulic system for construction equipment having a float function in the related art includes first and second hydraulic pumps 51 and 52 and a pilot pump 53; an operation lever (RCV) 58 which outputs an operation signal in proportion to an amount of operation; a float function switch (not illustrated) which selects a float function; a swing spool 54-4, an option spool 54-5, an arm spool 54-6, and a traveling spool 54-7 which are installed in a discharge flow path of the first hydraulic pump 51, and are shifted by pilot signal pressure from the pilot pump 53 according to the operation of the operation lever 58 to control hydraulic fluid supplied from the first hydraulic pump 51 to a swing device, an option device, an arm cylinder, and a traveling device, respectively; a boom spool 54-1, a bucket spool 54-2, and a traveling spool 54-3 which are installed in a discharge flow path of the second hydraulic pump 52, and are shifted by the pilot signal pressure from the pilot pump 53 according to the operation of the operation lever 58 to control hydraulic fluid supplied from the second hydraulic pump 52 to boom cylinders 55 and 55a, a bucket cylinder, and the traveling device, respectively; anti-drop valves 56 and 56a which are mounted on the boom cylinders 55 and 55a, respectively, to prevent the dropping of the boom; a solenoid valve 57 which is installed in a flow path between the operation valve 58 and the boom spool 54-1 to be shifted when the float function switch (not illustrated) is turned on; and a float valve 59 which is installed in a flow path between the boom spool 54-1 and the boom cylinders 55 and 55a, and is shifted by pilot signal pressure applied through the solenoid value 57 when the operation lever 58 is operated to make the boom descend in a state where the float function switch is turned on, so that the float valve 59 makes flow paths of large chambers and small chambers of the boom cylinders 55 and 55a communicate with each other to return the hydraulic fluid to a hydraulic tank.
A) A boom-down operation accompanied with no float function will be described.
In the case of operating the operation lever 58 to a boom-down side, the pilot signal pressure from the pilot pump 53 is supplied through a flow path 60, the operation lever 58, and a flow path 62, and is branched to flow paths 63 and 64.
The pilot signal pressure in the flow path 63 shifts spools of the anti-drop valves 56 and 56a, and the pilot signal pressure in the flow path 64 is supplied through the solenoid valve 57 (i.e. through the spool as illustrated in FIG. 1), and shifts the boom spool 54-1 in the right direction as shown in the drawing.
Accordingly, the hydraulic fluid discharged from the second hydraulic pump 52 passes through the boom spool 54-1, is discharged to a port A of the main control valve (MCV) 54, and then is supplied to the small chambers of the boom cylinders 55 and 55a. At this time, the hydraulic fluids which have returned from the large chambers of the boom cylinders 55 and 55a join together in a flow path 66 via the shifted spools of the anti-drop valves 56 and 56a. 
The hydraulic fluid in the flow path 66 is connected to the port A of the float valve 59 to be branched, is connected to a port B of the main control valve 54 via the shifted boom spool 54-1, and then returns to a hydraulic pump 74 via an internal path of the main control valve 54.
Accordingly, the boom cylinders 55 and 55a are contracted.
B) A boom-down operation accompanied with a float function will be described.
As shown in FIG. 1, when the float function switch is turned on, the solenoid valve 57 is shifted in downward direction as shown in the drawing by an electrical signal from the float function switch. Accordingly, in the case of operating the operation lever 58 to a boom-down side, the pilot signal pressure from the pilot pump 53 is supplied through the flow path 60, the operation lever 58, and the boom-down side flow path 62, and is branched to the flow paths 63 and 64.
As described above, the pilot signal pressure in the flow path 63 shifts the spools of the anti-drop valves 56 and 56a, and the pilot signal pressure in the flow path 64 is supplied through the solenoid valve 57 which has been shifted in the downward direction as shown in the drawing, and shifts a float spool 67 of the float valve 59 in the left direction as shown in the drawing.
At this time, a part of the hydraulic fluid from the second hydraulic pump 52 is supplied via the shifted boom spool 54-1, is discharged to the port A of the main control valve 54, and then is supplied to the small chambers of the boom cylinders 55 and 55a. At the same time, a part of the hydraulic fluid from the second hydraulic pump 52 is connected to the port B of the shifted float valve 59.
The hydraulic fluids which have returned from the boom cylinders 55 and 55a join together in the flow path 66 via the shifted spools of the anti-drop valves 56 and 56a. A part of the hydraulic fluid in the flow path 66 is connected to the port A of the float valve 59 which has been shifted in the left direction as shown in the drawing, and a part of the hydraulic fluid in the flow path 66 is connected to the branched port B of the main control valve 54 and returns to the hydraulic pump 74 via the shifted boom spool 54-1 and the internal path of the main control valve 54.
A part of the hydraulic fluid which has flowed into the port A of the float valve 59 joins again a part of the hydraulic fluid which have been supplied to the small chambers of the boom cylinders 55 and 55a, i.e. the hydraulic fluid which has flowed into the port B of the float valve 59 after passing through an orifice 68 formed in the float spool 67 of the float valve 59. The joined hydraulic fluid passes through an orifice 68a formed in the float spool 67, and returns to the hydraulic tank 74 via a tank line 69.
Accordingly, the boom cylinders 55 and 55a are contracted.
As described above, a part of the hydraulic fluid returning from the large chambers of the boom cylinders 55 and 55a directly returns to the hydraulic tank 74 through the port B of the main control valve 54. Also, a part of the hydraulic fluid returning from the large chambers of the boom cylinders 55 and 55a joins again the hydraulic fluid on the small chamber side of the boom cylinders 55 and 55a in the float valve 59, and then returns again to the hydraulic tank 74.
As described above, since the hydraulic fluid supplied to the small chambers of the boom cylinders 55 and 55a and the hydraulic fluid returning from the large chambers join together in the float valve 59 and are connected to the tank line 69 during the boom-down operation, the floating function depending upon the ruggedness state of the ground can be performed with low load pressure.
On the other hand, the load pressure can be adjusted in accordance with the size of the orifices 68 and 68a formed inside the float valve 59. In the case of the boom-down operation after the float function switch is operated, the hydraulic fluid of the hydraulic pump is controlled to be intercepted, and thus the boom descending and the float function by the weights of the boom cylinders 55 and 55a themselves can be performed.
As described above, the hydraulic system in the related art controls the three kinds of valves including the main control valve 54, a pair of anti-drop valves 56 and 56a, and the float valve 59 in order to perform the boom float function.
Due to this, the operability of the whole equipment should be evaluated by combining the respective valve control functions in matching the operational performance of the equipment, and thus it is difficult to control the equipment. Also, since the hydraulic line connection pipes are increased due to many kinds of valves, the manufacturing cost is also increased.